We present a comparison of aircraft measurements of halogenated very short
lived substances (VSLSs) and dimethyl sulphide (DMS, C2H6S)
from a co-ordinated campaign in January–February 2014 in the tropical west
Pacific. Measurements were made on the NASA Global Hawk, NCAR Gulfstream-V
High-performance Instrumented Airborne Platform for Environmental Research
(GV HIAPER) and UK Facility for Airborne Atmospheric Measurements (FAAM)
BAe-146 (see Sect. 2.2) using four separate gas chromatography–mass
spectrometry (GC-MS) instruments: one operated by the University of Miami
(UoM), one from the National Center for Atmospheric Research (NCAR) and two
from the University of York (UoY). DMS was measured on the BAe-146 and GV.
The instruments were inter-calibrated for halocarbons during the campaign
period using two gas standards on separate scales: a National Oceanic and
Atmospheric Administration (NOAA) SX-3581 standard representative of clean
low-hydrocarbon air, and an Essex canister prepared by UoM, representative of
coastal air, which was higher in VSLS and hydrocarbon content. UoY and NCAR
use the NOAA scale/standard for VSLS calibration, and UoM uses a scale based
on dilutions of primary standards calibrated by GC with FID (flame ionisation
detector) and AED (atomic emission detector). Analysis of the NOAA SX-3581
standard resulted in good agreement for CH2Cl2, CHCl3,
CHBr3, CH2Br2, CH2BrCl, CHBrCl2,
CHBr2Cl, CH3I, CH2ICl and CH2I2 (average
relative standard deviation (RSD) < 10 %). Agreement was in general
slightly poorer for the UoM Essex canister with an RSD of < 13 %.
Analyses of CHBrCl2 and CHBr3 in this standard however showed
significant variability, most likely due to co-eluting contaminant peaks, and
a high concentration of CHBr3, respectively. These issues highlight
the importance of calibration at atmospherically relevant concentrations
( ∼ 0.5–5 ppt for VSLSs; see Fig. 5 for individual ranges). The UoY in
situ GC-MS measurements on board the BAe-146 compare favourably with ambient
data from NCAR and UoM; however the UoY whole-air samples showed a negative
bias for some lower-volatility compounds. This systematic bias could be
attributed to sample line losses. Considering their large spatial
variability, DMS and CH3I displayed good cross-platform
agreement without any sampling bias, likely due to their higher volatility.
After a correction was performed based upon the UoY in situ vs. whole-air
data, all four instrument datasets show good agreement across a range of
VSLSs, with combined mean absolute percentage errors (MAPEs) of the four
platforms throughout the vertical profiles ranging between 2.2
(CH2Br2) and 15 (CH3I) % across a large geographic area of the tropical west Pacific. This study shows that the international VSLS calibration scales and instrumental techniques discussed here are in generally good agreement (within ∼ 10 % across a range of VSLSs),
but that losses in aircraft sampling lines can add a major source of
uncertainty. Overall, the measurement uncertainty of bromocarbons during
these campaigns is much less than the uncertainty in the quantity of VSLS
bromine estimated to reach the stratosphere of between 2 and 8 pptv
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